Mesoporogen-free synthesis of single-crystalline hierarchical beta zeolites for efficient catalytic reactions

Abstract

Single-crystalline hierarchical zeolites possess fast mass transfer, good active site accessibility, and enhanced hydrothermal stability for improved catalytic performance, which are highly desired in petro-, coal-, and fine-chemical industries. Here, we for the first time report the mesoporogen-free synthesis of single-crystalline hierarchical Beta nanozeolites, which is achieved by L-lysine-assisted regulation of zeolite growth kinetics in a two-step crystallization process. In this strategy, at low crystallization temperatures, L-lysine molecules could efficiently chelate with both silica and aluminum species, which leads to the aggregation of amorphous aluminosilicate gel particles in a non-compact manner towards forming interstitial pores. At subsequently elevated temperatures, intraparticle ripening becomes the predominant crystal growth behavior, where adjacent nanoparticles coalesce into larger ones coupled with the transformation of the initial interstitial pores into the final intracrystalline mesopores. The as-prepared hierarchical Beta nanozeolite (sample meso-Beta-23) shows a large surface area (617 m² g⁻¹) and a large mesopore volume (0.79 cm³ g⁻¹), as well as a single-crystalline feature, thus exhibiting improved properties in both gas- and liquid-phase catalytic reactions, including cracking of 1,3,5-trimethylbenzene to produce benzene, toluene, and xylene (BTX) (26.1% BTX selectivity); ethylation of benzene to produce ethylbenzene (73% ethylbenzene selectivity); and conversion of highly concentrated lactic acid (105 wt%) to produce lactide (73% lactide yield) compared to their conventional counterparts. Such high-quality single-crystalline hierarchical Beta nanozeolites may be promising for industrial applications in the conversion of various bulky feedstocks into value-added products.